This invention relates to closed circuit hydrostatic power transmissions comprising a variable displacement hydraulic pump and a hydraulic actuator connected to each other in a closed hydraulic circuit, and, more particularly, to a flushing valve system used in a closed circuit hydrostatic power transmission of, for example, hydraulic power shovels, which flushing valve system is operative in cooperation with fluid replenishing means, such as a charge pump, to replace the working fluid in the closed hydraulic circuit with a fresh supply of fluid.
Heretofore, a closed circuit hydrostatic power transmission is provided with a spool type flushing valve including two inlet ports each connected to one of two main lines of the closed hydraulic circuit, one outlet port connected to a working fluid reservoir or tank, and a spool having two ends each receiving a pressure from one of the two main lines and moved by the pressure differential to a position in which one of the two inlet ports is allowed to communicate with the outlet port. This type of flushing valve permits, when a predetermined pressure differential is produced between the two main lines, the inlet port connected to the main line of the higher pressure side to be blocked by the spool and the main line of the lower pressure side to be communicated with the tank, so as to discharge a portion of the working fluid in the main line into the tank.
In a spool type flushing valve of the prior art, it has been usual practice to have a large stroke of the spool, both to prevent the hydraulic motor from slipping due to pressure fluid leaks from the main line on the high pressure side through a gap between the body and spool of the flushing valve, and to increase the area of a passage between the inlet port communicating with the main line on the lower pressure side and the outlet port connected to the tank. Thus, the spool of the flushing valve of the prior art to has a long switching time when shifting from a first position, in which it allows the one inlet port to communicate with the outlet port, to a second position, in which it allows the other inlet port to communicate with the outlet port. It sometimes happens in a closed circuit hydrostatic power transmission that the internal pressures of the main lines suddenly show a change. For example, when pressure fluid is supplied from a hydraulic pump to a hydraulic motor connected to a load of high inertia to drive same, the main line connected to the discharge side of the hydraulic pump becomes a higher pressure side main line and the main line connected to the suction side thereof becomes a lower pressure side main line. If an attempt is made to suddenly decrease the volume of hydraulic fluid delivered by the hydraulic pump to stop rotation of the hydraulic motor, then the hydraulic motor would be driven by the inertia of the load connected to the hydraulic motor. This would result in the main line connected to the pump suction side that has been a lower pressure side main line becoming, for an instant, a higher pressure side main line and the main line connected to the pump discharge side that has been a higher pressure side main line becoming, for an instant, a lower pressure side main line. The flushing valve of the prior art has been unable to follow these sudden changes in the internal pressures of the main lines, so that even after the lower pressure side main line becomes a high pressure side main line, the main line still remains in communication with the tank through the flushing valve. Thus, working fluid of high pressure would be discharged into the tank and the hydraulic motor would continue to rotate without stopping.
Furthermore, in spite of the fact that the variable displacement hydraulic pump has decreased the volume of pressure fluid delivered thereby, the hydraulic motor tries to rotate at a velocity at which it has been rotating up to then, so that the pressure fluid flowing into the hydraulic motor would be reduced in volume and the pressure in the main line connected to the pump discharge side would become negative or subatmospheric, so that cavitation might develop and cause damage to the hydraulic motor. To avoid this accident, it has been usual practice to increase the volume of a charge pump for replenishing the main lines with pressure fluid to a level high enough to avoid the development of cavitation or to decrease the changing rate of the volume of pressure fluid delivered by the variable displacement hydraulic pump. However, an increase in the capacity of the charge pump is not desirable because it increases energy losses and increases limitations to be placed on designing. A decrease in the changing rate of the volume of pressure fluid delivered by the variable displacement hydraulic pump reduces its operational ability.